Is the canine corpus luteum an insulin-sensitive tissue?

An approach to uncover the relationship between 17b-estradiol and ESR1/ESR2 ratio in the regulation of canine corpus luteum

The canine corpus luteum (CL) is able to synthetise, activate and deactivate 17b-estradiol (E2) and also expresses nuclear estrogen receptors in a time-dependent manner during diestrus. Nevertheless, we are still missing a better comprehension of E2 functions in the canine CL, especially regarding the specific roles of estrogen receptor alpha (ERa) and ERb, encoded by ESR1 and 2, respectively. For that purpose, we analyzed transcriptomic data of canine non-pregnant CL collected on days 10, 20, 30, 40, 50 and 60 of diestrus and searched for differentially expressed genes (DEG) containing predicted transcription factor binding sites (TFBS) for ESR1 or ESR2. Based on biological functions of DEG presenting TFBS, expression of select transcripts and corresponding proteins was assessed. Additionally, luteal cells were collected across specific time points during diestrus and specificity of E2 responses was tested using ERa and/or ERb inhibitors. Bioinformatic analyses revealed 517 DEGs containing TFBS, from which 67 for both receptors. In general, abundance of predicted ESR1 targets was greater in the beginning, while abundance of ESR2 targets was greater in the end of diestrus. ESR1/ESR2 ratio shifted from an increasing to a decreasing pattern from day 30 to 40 post ovulation. Specific receptor inhibition suggested an ERa-mediated positive regulation of CL function at the beginning of diestrus and an ERb-mediated effect contributing to luteal regression. In conclusion, our data points toward a broad spectrum of action of E2 and its nuclear receptors, which can also act as transcription factors for other genes regulating canine CL function.

Insulin induces steroidogenesis in canine luteal cells via PI3K-MEK-MAPK

Glucose uptake increases in canine luteal cells under insulin treatment. We hypothesize that insulin also increases luteal cell steroidogenesis. Dogs underwent elective ovariohysterectomy from days 10-60 post ovulation and their corpora lutea (CL) and blood samples were collected. Deep RNA sequencing determined differentially expressed genes in CL; those related to insulin signaling and steroidogenesis were validated in vivo by qPCR and their respective proteins by Western blotting and immunofluorescence. Next, luteal cell cultures were stimulated with insulin with or without inhibition of MAPK14, MAP2K1 and PI3K. Studied proteins except P450 aromatase showed the same expression pattern of coding genes in vivo. The expression of HSD3B and CYP19A1 was higher in insulin-treated cells (P < 0.005). Following respective pathway blockades, the culture medium had decreased concentrations of progesterone (P4) and 17b-estradiol (E2) (P < 0.01). Our results indicate that insulin increases HSD3B and CYP19A1 expression via MAPK and PI3K, and contributes to the regulation of P4 and E2 production in canine luteal cells.

The involvement of hypoxia-inducible factor 1α (HIF1α)-stabilising factors in steroidogenic acute regulatory (STAR) protein-dependent steroidogenesis in murine KK1 granulosa cells in vitro

As a component of hypoxia-inducible factor1 (HIF1)-complexes, HIF1α regulates the expression of steroidogenic acute regulatory (STAR) protein in granulosa cells. However, severe hypoxia or exaggeratedly expressed HIF1α have detrimental effects. HIF1α is regulated by factor inhibiting HIF (FIH), prolyl hydroxylases (PHD1, 2, 3) and von Hippel-Lindau (VHL) suppressor protein. In this study, the expression of FIH, PHD1, 2, 3 and VHL was investigated in murine ovaries and immortalised KK1 granulosa cells. We found FIH, VHL and PHD2 transcripts predominantly in growing tertiary follicles. Functional aspects were assessed in KK1 cells exposed to decreasing O2 (20%, 10%, 1%), by determining HIF1α, FIH, VHL, PHD1-3 and STAR expression. The main findings indicated gradually increasing PHD2 under lowered O2. Functional blocking of PHDs revealed biphasic effects on STAR expression; concomitantly with increasing HIF1α, STAR expression, which was initially induced, decreased significantly when HIF1α was strongly stabilised. Finally, PHD2 in particular might act as a specific regulator of HIF1α and, thereby, of STAR availability in granulosa cells.

HIF1 driven transcriptional activity regulates steroidogenesis and proliferation of bovine granulosa cells

Hypoxia-inducible factor 1 (HIF1) is a heterodimeric transcription factor, consisting of a constitutively expressed β-subunit (HIF1B) and a regulated α-subunit (HIF1A). In the present study, we analyzed the HIF1 driven transcriptional activity in bovine granulosa cells (GC). Treatment of GC with FSH (follicle stimulating hormone) and IGF1 (insulin-like growth factor 1) resulted in the upregulation of HIF1A mRNA expression under normoxia. Immunohistochemistry of bovine ovarian sections showed distinct staining of HIF1A in the GC layer of different staged ovarian follicles. Suppression of HIF1 using echinomycin and gene knockdown procedures revealed that HIF1 transcriptionally regulates the genes associated with steroidogenesis (STAR, HSD3B and CYP19A1) and proliferation (CCND2 and PCNA) of GC. Further, our data suggest that CYP19A1, the key gene of estradiol production, is one of the plausible downstream targets of HIF1 in bovine GC as shown by gene expression, radioimmunoassay, and chromatin precipitation analysis. Based on these results, we propose that HIF1 driven transcriptional activity plays a crucial role in GC functionality, especially steroidogenesis and proliferation in developing bovine ovarian follicles.

Factors affecting the fate of the canine corpus luteum: Potential contributors to pregnancy and non-pregnancy

The fate of the canine corpus luteum (CL) differs from that of other domestic species: beyond the extended luteal regression observed in both pregnant and non-pregnant cycles, active luteolysis is observed only in pregnant dogs. Luteal regression in the absence of pregnancy lacks a luteolytic trigger. The CL lifespan during pregnancy is around 60 days, as long as that of the cyclic CL. Although they are already available in the first half of diestrus, LH and especially prolactin (PRL) play a decisive luteotropic role from approximately day 25 post-ovulation onwards. Nevertheless, many locally-produced factors are orchestrated to ensure a fully functional CL, which in the bitch produces progesterone (P4), 17b-estradiol, and other local regulators. Recently, insulin has been described as another luteotropic factor in this species, able to increase glucose uptake in luteal cells and contribute to steroid biosynthesis. The locally-produced PGE2 is also a potent luteotropic factor in the first half of diestrus, promoting STAR expression, as are also proliferating, vasoactive- and immunomodulatory factors. These, in turn, all contribute to the formation and maintenance of the canine CL. Meanwhile PGF2a, produced by the utero-placental compartment, participates actively in triggering pre-partum luteolysis. Cytokines play different roles, either contributing as luteotropic or as acute inflammation molecules. So far, the one clinically most efficient mechanism of interrupting a pregnancy in the dog is to block P4 receptors, using an antigestagen (e.g., aglepristone) in the second half of diestrus. To enhance the chances of pregnancy, however, several luteotropic factors could be used.